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How are engineer’s straight edges made?

How are engineer’s straight edges made?

Shop for Engineer’s Straight Edges

Steel, cast iron, and aluminium straight edges

Manufacturing of metal parts The main processes that steel straight edges can go through to make them better suited to their job are: heat treating, tempering, scraping, grinding and lapping. Cast iron straight edges are often cast to achieve the general shape required, then will have their working surfaces finished by scraping, grinding or lapping.
Aluminium being extruded from a machine Aluminium is frequently extruded, as this can be a very quick and cost-effective way to make objects. However, an extruded aluminium straight edge will require finishing similar to cast iron straight edges that are cast in order to achieve the accuracy needed for the working surface.
Pouring the molten metal into a mould during casting


Casting is a manufacturing process that involves pouring a molten metal into a mould where it cools and forms to the shape of the mould. Many intricate shapes can be made this way.

Casting can reduce or in some cases eliminate the amount of machining a part requires. It is most commonly carried out with iron although it’s also possible to cast steel and aluminium.

Quenching a workpiece during the heat treating process

Heat treatment

Heat treating and tempering are manufacturing processes used to alter the physical properties of metal and other materials.

Heat treating involves heating the metal to a very high temperature and then quenching (rapidly cooling) it. This increases the hardness of the metal, but in doing so will also make it more brittle.

Tempering a workpiece to reduce the brittleness


Tempering is performed after heat treating, and also involves heating the metal, but to a lower temperature than required in heat treating, then letting it cool slowly. Tempering decreases the hardness and brittleness of the metal whilst increasing its toughness. By controlling the temperature the metal is heated to during tempering, it’s possible to alter the final balance between the hardness and toughness of the metal.

Aluminium extrusion process, Billet, Die, Ram, Extrusion, aluminium extrusions


Extrusion is a compression forming manufacturing technique where a material is shaped by a ram forcing the metal through a die. The die is shaped to produce the required cross sectional shape for the finished workpiece. Aluminium is by far the most common material used in extruded manufacturing.

Granite straight edges

Large granite block being cut to size with a water cooled circular saw Granite engineer’s straight edges are first roughly cut from a large block of granite. This is done using large water cooled saws.

Once the general shape has been attained, the finish and accuracy required for use as an engineer’s straight edge is achieved by grinding, scraping or lapping.

Grinding the surface of an engines cam


Grinding is the process of using a bonded grinding wheel made of abrasive particles to remove material from a workpiece. The grinding wheel is a disc that is spun at high speed, and the workpiece is passed across the side face or circumference surface.

Grinding can be done with discs made with grain sizes ranging from 8 (course) to 250 (very fine). The finer the grain size, the better the surface finish on the workpiece will be.

Hand scraping to achieve a flat surface on the workpiece


Scraping is the process by which the surface of a workpiece is scraped of high spots in order to produce a finished surface that is flat. Scraping can be performed on any metal part that requires a flat surface.

Lapping uses an abrasive paste between the workpiece and the lapping tool to wear flat the workpiece


Lapping is a finishing process used in manufacturing to achieve a smoother and flatter surface on a finished product. Lapping involves a lapping compound made up of abrasive grits and oils placed between the surface of a workpiece and a lapping tool. The lapping tool is then moved over the surface of the workpiece.

Lapping compound is a paste containing fine grit particles that wear away at the surface of a workpiece The abrasive nature of the lapping compound wears away imperfections in the workpiece surface and produces an accurate and smooth surface finish. The most common types of abrasives used in lapping are aluminium oxide and silicon carbide, with a grit size between 300-600.

Grinding, scraping or lapping?

What process produces the most accurate straight edge, grinding scraping or lapping? Grinding does not produce as flat a surface as lapping or scraping. Scraping can only be done on metal workpieces, so cannot be used for the production of granite straight edges.

The size of the straight edge will determine whether scraping or lapping will produce the better quality straight edge. As a general rule, scraping is more accurate than lapping over long lengths, but the only way to tell for definite which straight edge is going to be more accurate is to look at the manufacturers’ tolerances for the engineer’s straight edges you are considering purchasing.

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